StatsComponent perf improvement for min, max, and situations where all stats disabled

as mentioned in SOLR-6349, when i started doing perf testing of requesting individual stats, i noticed that min (and it turns out max) were slower to compute then more complex stats like sum & mean.

While investigating, i realized that we can also optimize the case where a stats.field param is specified, but no stats are computed for example: stats.field=

fieldname&doMin=false